Astronomical Peek-a-Boo

© 2001 by Robert C. Moler

Some of the most spectacular events seen in the sky occur when one celestial body hides another. The most spectacular of all is the total solar eclipse. However the total solar eclipse gets its beauty because the moon doesn't hide all of the sun. It just screens the sun's intense face allowing us to behold the fainter wonders of the sun's chromosphere and corona.

When the moon covers a lesser body, in brightness that is, the event is called an occultation. Occultation comes from the word occult, which means to hide. We astronomers use the word occult as a verb. The moon can occult stars and planets, while planets and asteroids can occult stars.

While astrologers make a big deal of planetary alignments that may or may not include the earth, I prefer alignments that I'm part of, be it solar eclipses or an occultation. The closest I got to an exact alignment was in July 10, 1972 at mid eclipse during totality of a solar eclipse. Our observing spot on Prince Edward Islands was half a mile from being exactly in line with the center of the 865,000 mile diameter sun and the center of the 2,100 mile diameter moon.

Aside from the privilege of being in line with great celestial bodies, timing such events has practical importance. The timing of a star's appearance or disappearance at the edge of the moon from a known location on the earth is used to accurately determine the position of the moon. It takes many occultation observations of many stars by many observers to accurately determine the orbit and true motion of the moon.

From observations like these astronomers found out that the moon kept a different time than the earth. Earthly time is based on the rotation of the earth. But, it turned out that the earth is slowing down. So time based on that changing rate cannot be used for time based position calculations of the sun, moon and planets. Atomic clocks are also immune to the earth's slowing rotation. Now, atomic time has become the basis for earthly time. So to keep our clocks a tuned to the sun via the earth's rotation, an occasional leap second is added at approximate 18 month intervals. The actual rate of slowing is irregular and can only be accurately determined by observation.

When a star is occulted by the airless moon it simply winks out, since a star's actual diameter is so tiny. The moon occults several dozen bright stars a month, and since the moon's orbit processes or wobbles, it cuts a wide swath of sky over time. First Magnitude stars that can be occulted by the moon are Aldebaran, Regulus and Spica. The moon's occultation of members of the Pleiades is an eagerly anticipated event. The moon also occults the planets, and the Crab Nebula, a supernova remnant also known as M1. Occultations were able map the Crab's X-ray image in the days before X-ray telescopes could resolve such detail.

A very interesting type of lunar occultation is the grazing occultation, at the north and south limits of occultation visibility. The path where an occultation is visible on the earth is the width of the moon or greater because the shadow of the moon falls on a spherical earth. At the extremities of the path the star can be seen disappearing behind peaks reappearing in the valleys of the moon's rugged limb. By stationing observers along a north-south line a map of the moon's edge can be made.

Back in the 1970's NASA's old Kuiper airborne observatory, a 36 inch telescope mounted in a C-141 aircraft, was flying over the Indian Ocean to observe the occultation of a star by the planet Uranus. The intent was to get a more accurate measurement of the diameter of the planet. However in tracking the star in towards the planet, the star winked out several times before it even got to the edge of the planet. Again after the occultation the star winked out again several times. It was obvious that Uranus was surrounded by several unseen rings. The rings were subsequently observed by ground telescope techniques and seen up close by the Voyager 2 spacecraft in 1986.

The newest type of occultation observations are asteroidal. These are useful in determining the size and shape of asteroids. One instance of a possible satellite of an asteroid was discovered this way. This was before the Galileo spacecraft discovered one around the asteroid Gaspara it passed close to on its way to Jupiter.

What is needed to accurately time occultations? In order for occultation timing to mean anything, ones location (latitude, longitude, and altitude above sea level) must be known accurately. Then the time of the event must be known. A tape recorder recording the short wave WWV time signals and the observers voice is an easy way to record the event. An allowance for the observer's reaction time is also made. Bright star occultations can also be made with the newer, super light sensitive video cameras, taking human reaction time out of the equation.

There is an organization that prepares charts and coordinates observations of asteroidal occultations. It is the International Occultation Timing Association, or IOTA fort short, run by David Dunham. Their web site is

This month's meeting will explore occultations and eclipses in more depth.

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Uploaded: 02/25/01